1. Technical Field
The present invention relates to integrated circuits, and more particularly to field effect transistor based sensing devices having an extended gate structure.
2. Description of the Related Art
The measurement of pH is important in many chemical and bio-chemical reactions. Sensors that measure pH may have multiple applications and in particular may provide measurements in processes, such as, DNA sequencing, enzymatic reactions where protons are produced, e.g., glucose detection, etc.
In many instances, pH sensors are included in integrated circuits. A floating gate field effect transistor (FET) pH sensor may be employed for sequencing DNA. A bead is coated with multiple copies of single DNA strands; nucleotides are flowed into a space or a well above a floating gate. The well includes dielectric walls and a metal oxide dielectric sensing layer. When a polymerization reaction occurs, protons are released into the well thus causing transient pH to change in the well. The pH change is sensed by the floating gate of the sensor, which is below the metal oxide sensing dielectric layer. The metal oxide dielectric layer is used as the pH sensing surface in contact with the solution.
Since the pH sensing surface is dielectric, only the bottom of the well is employed as the sensing surface. The sensing surface is limited by layout area restrictions, and the floating gate design and the pH (i.e., proton concentration) signal is limited based on the available sensing capabilities. As a result the pH sensitivity decreases.